Well Fargo urged its customers on Thursday to visit bank branches or use telephone banking due to continuing problems with its website.
In a statement, a bank spokeswoman described the issue as "an unusually high volume of traffic" that is causing slow or intermittent access to its website.
"The vast majority of customers are not impacted, but for those who are, we encourage them to access their accounts through our stores, ATMs or by phone as we work to resolve the issue," she wrote.
Wells Fargo is one of several banks that have been targeted with distributed denial-of-service (DDOS) attacks over the last several months, which seek to jam websites with traffic to make them unavailable.
A group calling itself the "Izz ad-Din al-Qassam Cyber Fighters" launched attacks against Wells Fargo and other financial institutions in September following the publication of a 14-minute video trailer insulting the Prophet Muhammed. Wells Fargo apologized at the time for the disruptions.
The video, which appeared on YouTube, has been removed by Google in some countries since it violates local law, but the company has left it up in most countries since it does not violate its guidelines.
The Izz ad-Din al-Qassam Cyber Fighters Group posted a message on Pastebin on Tuesday, saying that it intends to continue to launch attacks against financial institutions until the video is removed.
It warned this week's attacks will be on the same scale as last week. Arbor Networks, a networking security company, said some of the DDOS attacks peaked at 60Gbps, up from an average attack rate of just 1.67 Gbps in September. The attackers are employing a tool called Brobot that sends specially crafted DNS (Domain Name System) packets, the company said.
Other banks targeted include U.S. Bancorp, JPMorgan Chase, Bank of America, PNC Financial Services Group and SunTrust Banks.
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